New No-biofix Degree-Day Model for Codling Moth

What is biofix?

"Biofix is an easy to observe event (example: first moth capture) that was previously used to synchronize the model and field populations. " Dr. Vincenet Jones, Prof./Res. Entomology, WSU -TFREC

The codling moth model is the model most people think of when they think of biofix. In 2008 the WSU Decision Aid System (DAS) began the process of transisioning away from using a biofix for the codling moth model. After comparing 32 orchard-years worth of data taken at the WSU research station in Wenatchee and Columbia View with 81 orchard-yeaars worth of data from consultants throughout the state of Washington, it was found that biofix occured on average at 175 (°F) Degree Days (DD) from January 1. They then compared model predictions to observed adult flight and egg hatch and found that there were no benefits in terms of accuracy using a biofix. The codling moth model on DAS therefore automatically sets a biofix at 175 DD from 1 January. In the past, we would reset the model DD to 0, then accumulate the heat units from that point. However, this is counter-intuitive and confusing; for example 160 DD will occur twice during the same season and likely within 40 days of each other. Instead of using the old convention of resetting the biofix, DAS isnow simply accumulating degree-days since 1 January. However, for people used to the old system, our references will also report in parenthesis, the older number. For example, the start of egg hatch is at 425 DD from 1 January (250 DD old model). When viewing the codling moth model in DAS you will see this used in the predictions and recommendations portions of the output.

The codling moth model used in Washington is probably fine in areas more northerly than Washington State, but definitely should not be used in more southern areas. Dr. Jones is in the process of collecting biofix timing information throughout North America and preliminary results show that the timing of first moth is related to temperature and solar radiation, but above 46°N latitude it levels off at roughly 175 DD. After further work, corrections will be built into DAS to account for first moth in areas outside the state.

What are degree-days and why are they used in the models?

Degree-days (DD) are used in models because they allow a simple way of predicting development of cold-blooded organisms (insects, mites, bacteria, fungi, plants). The idea is that the development of cold-blooded organisms depends on the temperatures that they experience in the environment, because they cannot regulate their temperature to any significant degree. It has been shown that the biochemical processes involved in development through a particular stage (such as the egg or larval stages) require a given amount of time and that the amount of time varies depending on the temperature the organism experiences. Work in the early 1920’s showed that there was a temperature below which development did not occur (= lower threshold for development) and above a certain temperature development rapidly slowed (= upper threshold for development). In between the lower and the upper thresholds, the developmental rate was a straight-line function of the temperature. This meant that development could be predicted by knowing how long the organism was held at any temperature between the thresholds. Another way of saying this is that the amount of heat units required to complete development was constant – a finding that is the basis for the models used on DAS.

Formally, the heat units are termed degree-days (DD) and by definition a DD is the amount of heat that accumulates when the temperature is 1° above the lower threshold for development for a period of 24 hours. Each organism has a different lower threshold for development, which makes comparisons sometimes more difficult. For the insects on the DAS system, each of them have a slightly different upper and lower thresholds for development.

For most of the models on DAS, we use a single sine wave fit to the daily maximum and minimum temperatures to determine the DD accumulations. There are three ways commonly used to modify the sine wave. The simplest form uses no upper threshold for development and this is used for western cherry fruit fly and apple maggot. In this case the heat units are calculated by the computer as simply the entire area under the temperature curve (Fig. 1). For a codling moth, peach twig borer, and Lacanobia fruit worm, we use a horizontal cutoff, which eliminates the heat accumulations that occur above the threshold (Fig. 2). In effect, this approximation suggests that the development rate is constant above the upper threshold. While this doesn’t seem logical given the discussion above, it works in practice pretty well. The other type of cutoff is a vertical cutoff, where no heat units are accumulated once the temperature goes above the threshold (Fig. 3). This type of cutoff is used for Pandemis leafroller, Obliquebanded leafroller, Campylomma bug, and San Jose Scale.

To view the degree-day development table for codling moth showing both DD-with biofix and DD-no biofix values click here.